Characterization of Raloxifene Glucuronidation in Vitro: Contribution of Intestinal Metabolism to Presystemic Clearance

Abstract

This article is available online at http://dmd.aspetjournals.org ABSTRACT:Raloxifene, a selective estrogen receptor modulator used for the treatment of osteoporosis, undergoes extensive conjugation to the 6-␤-and 4-␤-glucuronides in vivo. This paper investigated raloxifene glucuronidation by human liver and intestinal microsomes and identified the responsible UDP-glucuronosyltransferases (UGTs). UGT1A1 and 1A8 were found to catalyze the formation of both the 6-␤-and 4-␤-glucuronides, whereas UGT1A10 formed only the 4-␤-glucuronide. Expressed UGT1A8 catalyzed 6-␤-glucuronidation with an apparent K m of 7.9 M and a V max of 0.61 nmol/min/mg of protein and 4-␤-glucuronidation with an apparent K m of 59 M and a V max of 2.0 nmol/min/mg. Kinetic parameters for raloxifene glucuronidation by expressed UGT1A1 could not be determined due to limited substrate solubility. Based on rates of raloxifene glucuronidation and known extrahepatic expression, UGT1A8 and 1A10 appear to be primary contributors to raloxifene glucuronidation in human jejunum microsomes. For human liver microsomes, the variability of 6-␤-and 4-␤-glucuronide formation was 3-and 4-fold, respectively. Correlation analyses revealed that UGT1A1 was responsible for 6-␤-but not 4-␤-glucuronidation in liver. Treatment of expressed UGTs with alamethicin resulted in minor increases in enzyme activity, whereas in human intestinal microsomes, maximal increases of 8-fold for the 6-glucuronide and 9-fold for the 4-glucuronide were observed. Intrinsic clearance values in intestinal microsomes were 17 l/min/mg for the 6-glucuronide and 95 l/min/mg for the 4-isomer. The corresponding values for liver microsomes were significantly lower, indicating that intestinal glucuronidation may be a significant contributor to the presystemic clearance of raloxifene in vivo.

Automation and Miniaturization of the Bioluminescent UGT-Glo Assay for Screening of UDP-Glucuronosyltransferase Inhibition by Various Compounds

“…In that regard, the similarity between the indinavir IC 50 measured here and its reported K m are expected in that both values reflect its affinity for the UGT1A1 active site. 14 There was very little inhibition of UGT2B7 by either drug (Fig. 3C, D), consistent with reported IC 50 values above 100 mM.…”

New insights into the metabolism of tamoxifen and its role in the treatment and prevention of breast cancer

“…Only about 2% of administered raloxifene is bioavailable [1] but despite this, the drug is known to have a long biological half life of 27 hr. The reason for this disparity is that raloxifene is a polyphenolic drug that can be glucuronidated and sulfated by bacteria in the gut so the drug cannot be absorbed [107,108]. This phase II metabolism in turn controls enterohepatic recirculation and ultimately impairs the drug from reaching and interacting with receptors in the target.…”

Phase II Drug Metabolism

“…The intestinal UGT1A8 and UGT1A10 enzymes were suggested to have a negative impact on the bioavailability of orally administered therapeutic drugs (Mizuma, 2009). For example, raloxifene, a selective estrogen receptor modulator used in therapy of osteoporosis, naturally has a low bioavailability and has also been shown to be extensively metabolised by UGT1A8 and UGT1A10 (Kemp et al, 2002). UGTs might also play a significant role in the inactivation of carcinogens from diet or cigarette smoke (Dellinger et al, 2006).…”